1. Field of the Invention
This invention is directed to a unity gain filter for a current transformer, and more particularly to such a filter utilized by an electrical device, such as an electrical contactor, for filtering the output of a di/dt toroidal current transformer and maintaining current sensing accuracy for a chopped silicon controlled rectifier (SCR) current waveform sensed by the current transformer.
2. Background of Information
Toroidal transformers are generally utilized to sense current flowing in a power line conductor. In particular, di/dt current transformers are used to sense changes in such current. The di/dt current transformers operate by sensing changes in flux in their core and produce an output signal that is proportional to the change in current flowing through the core (i.e., a derivative of the core current). Under conventional operation, with alternating current having a generally sinusoidal (sine) waveform, the derivative of the alternating current is also a generally sinusoidal (cosine) waveform. Thus, conversion of the output signal of the current transformer to a value representing the alternating current is relatively straightforward using well-known analog rectification or integration and analog-to-digital conversion techniques.
However, whenever the rate of change of the alternating current is large, such as the case where higher frequency harmonics are present in the alternating current waveform, then the current transformer output signal is also large. In this case, the conversion of the current transformer output signal to a value representing the true alternating current is not directly possible. This is because the current transformer does not produce an accurate sinusoidal output signal in relation to the input alternating current and, instead, distorts the true alternating current waveform. Such distortion is likely in a power system having a chopped SCR alternating current waveform. This is because the chopped SCR waveform is not sinusoidal and contains many higher frequency harmonic components.
There is a need, therefore, for an electrical circuit for a current transformer that maintains current sensing accuracy for currents sensed through the transformer.
There is a more particular need for an electrical circuit for a di/dt current transformer that maintains current sensing accuracy for a chopped SCR current waveform.
There is another more particular need for an electrical circuit for a di/dt current transformer that maintains current sensing accuracy for a current waveform having third, fifth and seventh order harmonics.